Ink jet synchronization and failure detection system

ABSTRACT

Individual ink droplets are formed at a nozzle, propelled toward a record medium, variably charged by a charge electrode, and deflected by deflection plates for printing of characters. Droplets not required for printing are directed to a first gutter. The charges on the droplets develop a current in the first gutter that is sensed in an electronic feedback loop for synchronization of drop formation with drop charging. An auxiliary gutter is positioned to receive drops during a checking interval when a relatively high charge is applied to the unused drops. The relatively high charge on the drops develops a much larger current in the auxiliary gutter that is easier to detect. If desired, synchronization can also be effected by using the auxiliary gutter.

Hill et a1.

INK JET SYNCHRONIZATION AND FAILURE DETECTION SYSTEM [111 3,769,630 Oct.30, 1973 Primary Examiner-Joseph W. Hartary 751 Inventors: James D.Hill; Hugh E. Naylor, III; Kendal] gznald L. West, all of Lexington,ABSTRACT Individual ink droplets are formed at a nozzle, pro- [73]Asslgnee: Intel-natural Busmess Machmes pelled toward a record medium,variably charged by a corporaton Armonk, charge electrode, and deflectedby deflection plates [22] Filed; June 27, 1972 for printing ofcharacters. Droplets not required for printing are directed to a firstgutter. The charges on [21] Appl. No.: 266,790 the droplets develop acurrent in the first gutter that is sensed in an electronic feedbackloop for synchroniza- 52 0.5. CI. 346/75 tion of drop formation with pcharging- An auxil- 51 rm. C1. G0ld 18/00 iary gutter is Positioned toreceive drops during a [58] Field of Search 346/75; 317/3 checkinginterval when a relatively high charge is pplied to the unused drops.The relatively high charge 5 References Cited on the drops develops amuch larger current in the UNITED STATES PATENTS auxiliary gutter thatis easier to detect. If desired, synchronization can also be effected byusing the auxil- 3,562,761 2/1971 Stone et a] 346/75 iary gutter.3,596,276 7/1971 Lovelady 3,681,778 8/1972 Keur 346/75 6 Claims, 4Drawing Figures t :j :f i 7 u F MASTER MACHINE M ,W

CLOCK 13 LOGIC 11 M l I CHARACTER GENERATOR lo 9 .L (52 44 46 ANALOGSWITCH Ajf (BUTTER "E l SELECTION) V i MOUNTING MEANS 45b MOVING MEANSPATENTEDnmo ms 3; 769.630

' SHEET 10F 2 "wmm" j T MASTER MACHINE 1 CLOCK LOGIC CHARACTER -aGENERATOR 4o ;SYN9.-EU. E: 'RAMP-i g 5 e2 SYNC -l 5 CONTROL CHEW/455i 4e5 glim 5 i ANALOG SGTCH A 65 5 v I 54 I E I (GUTTER B 5 SELECTION)REFERENCE 52 g VLTAGE1 5 l i Y 55 I 60 COMPARATOR 0 O 0 Q o h 31 45039135 2G v a i t r ,1 /39a CUTTER N01 L J L I 38 MQUNTlNG MEANS I b MOVINGMEANS. SUPPLY is 5 27 200o'do oi GUTTER N0. 1

PAIENIEflnmamszs 3 9 6 SHEET ear 2 1011s I DROP TIME J L FIG. 3

NORMAL CHARGE ELECTRODE DRIVER SETTLING TIME ilfins f fius h "3.511s *3-10 VOLTS 7 sum PULSE GUTTER N0.1 m VOUS s g 1o vous SYNC RAMP GUTTERN0.1 L--ov0us i 2$0 VOLTS 5 --2oovoLT SYNC RAMP GUTTER N0.2 5 w VOLTS r5 v 5 --VOLTS CHECK|NGPULSEGUTTERNO.1 g; 1...

Y I -2oovous CHECKING PULSE CUTTER N0.2

SAFE AREA OF DROP FORMATION FIG. 4

5 L87. MASTER PR|NT|NG PRINT SIGNAL CLOCK. LOGIC (82 9e INK SUPPLY ORSYNC PULSE 85 INK SUPPLY REF VOLTAGE no a4 SYNC COUNTER DECODE CONTROLREGISTER SELECTOR INK JET SYNCHRONIZATION AND FAILURE DETECTION SYSTEMBACKGROUND OF THE INVENTION AND PRIOR ART -The following US. Pat. Nos.are representative of the prior art: Lewis, et al, 3,298,030; Keur, etal, 3,465,350; Keur, et al, 3,465,351; Lovelady, et a], 3,596,276; andSweet 3,596,275.

The Lewis, et al, patent describes drop synchronization using a phaseshifter to insure proper charging of drops at the correct time. TheKeur, et al, U.'S. Pat. No. 3,465,350 describes the use ofa video pulsethat is applied to the ink drops, and if the phase is correct, a dropdetector is actuated to so indicate. The Keur, et al, U.S. Pat. No.3,465,351 also makes use of a drop detector in various arrangements. TheSweet patent describes ink jet writing apparatus incorporating means forvariably charging the drops and maintaining a constant potential on thedeflection plates. The Lovelady patent illustrates a form of servo loopfor controlling the phase of the charging voltage and making use of adrop catcher.

None of the foregoing art describes a system like the present systemmaking use ofa pair of drop catchers or gutters and the development ofrelatively high currents and signal levels not normally encountered. inthe sys- -tem during printing of characters.

SUMMARY OF THE INVENTION A In accordance with the present invention, asystem incorporates means for producing ink drops as a stream of drops,facilities for variably charging the droplets in accordance withinformation or for synchronization and checking purposes,deflectionmeans for deflecting the variably charged drops toward paperor either one of a pair of gutters, in dependence upon the amount ofcharge on thedrops, and circuitry for detecting proper phasing orsynchronization of drops with respect to charging and formation ofdrops, as well as failure detection. In its various forms, the inventioncontemplates an electronic servo or feedback loop operable in a realtime fashion to maintain proper synchronization.

In one version, a counter means is advanced until a change current aboveapredefined level is found. The synchronization procedures are combinedin one embodiment with a failure detection capability making use of asecond gutter that receives drops when properly phased and that developshigher currents that are easier to detect. The second gutter may also beused for synchronization purposes if higher charge potentials areapplied.

OBJECTS Accordingly, a primary object of the present inven-, tion is toprovide a system for ink jet printing apparatus to determine propersynchronization of drops formed in the system.

Still another object of the invention is to provide a system havingsynchronization and failure detection ca-' tablished to check properdrop formation and phasing.

Another object of the invention is to provide a system that is operableto detect loss of electric field, loss of charge voltage, and relatedfailures in an ink jet apparatus.

Another object of the invention is to provide monitoring of an ink jetsystem while printing, between characters or while in the home positionetc., by means so as to immediately detect a failure in synchronization.

A further object of the present invention is to enable synchronizationon a line-by-line basis, as when the printing means is at a homeposition, or between characters during actual printing operations, orwithin the matrix boundaries of an individual character when unuseddrops are available for checking purposes.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the various embodiments of the invention as illustratedin the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a diagram of an ink jet printing systemincorporating the synchronizing and checking techniques of the presentinvention and particularly including a pair of gutters, and variouscircuitry controlling printing, synchronizing, and checking.

FIG. 2 is a modified comparing arrangement for use i in the circuit ofFIG. 1.

DETAILED DESCRIPTION Ink Jet Printing System The system of FIG. 1 isprimarily intended for printing of intelligence, such as characters orsymbols on a record member, such as a sheet of paper 1, by formation,propulsion, charging, deflecting, and deposition of ink drops 2 on paper1, supported by means 4. The drops move in a stream or streams at highspeeds from a source' 3. Ink from an ink supply 5 is directed by meansof pump 6 to source 3 which incorporates a vibrating means, such as apiezoelectric crystal 8 and an associated nozzle 10. A master clock 11provides basic timing pulses to the system including machine logic 13and a character generator block 14. Crystal 8 is driven at the frequencyprovided by clock 11 under control of crystal driver 15.. The frequencymay be a very high range such as kilohertz, or more. Pulsation bycrystal 8 effects formation of individual droplets that are directedthrough the center of a charge electrode 18 as is known in the art.Characters on paper 1 may be formed as a matrix of droplets, forinstance, 24 droplets wide by 40 droplets high. In order to control theplacement of drops on paper 1, a varying amount of charging voltage isprovided to charge electrode 18 from charge electrode driver 21.Individual drops are directed between deflection plates 22 and 23 havinga high potential, such as 3,000 volts, supplied from terminal 25. As istaught in the art, the arrangement is such that a constant potentialexists between plates 22 and 23 which when combined with the variablecharging on drops 2 effects selective displacement of the drops in avertical sense, for example, to any one of the 40 possible positions inthe, matrix. Unused drops are directed either to gutter No. 1,designated 35, or togutter No. 2, designated 36, as will be described ingreater detail. These drops are returned by lines 27-and 27a undercontrol of pump 30 to ink supply 5. The proper voltage applied to drops2 by charge electrode 18 from driver 21 during printing of charactersis'supplied by line 32 from character generator 14. Ordinarily, a singlevertical column of drops is propelled toward paper 1 with selectivedeflection onto the paper at appropriate positions or into one of theother-of the gutters 35 or 36. Source 3 and elements 8, .10, 18, 22-23,35 and 36 are customarily mounted on a mounted means 37 interconnectedwith said elements bydashedlines 39a and 3922. Formation of a pluralityof columns in a horizontal direction, such as the exemplary number of 24is effected by relatively moving paper l and source 3, as well aselectrodes 18, 22, and 23 in a timed fashion to achieve a side-by-sidearrangement of columns, as by moving means 38 interconnected to mountingmeans 37 by line 45a and to support means 4 by line 45b. Such movementmay be effected on an incremental basis or on a continuous basis. Inthis manner, entire lines on a document are printed. Ordinarily, at theend of each line of printing, the ink drop generating and deflectingmeans is relatively displaced with respect to paper 1 to a home positionin preparation for a succeeding line. At the home position, as wellasbetween characters, during actual character formation and whilereturnin g'to home position, the various synchronizing and checkingprocedures set forth herein may be employed to control drop formation,and the-relative timing of charging by electrode 18 with respect toindividual drops passing through electrode 18. v V

AUTOMATIC SYNCI-IRONIZATION OF DROP SEPARATION AND CHARGE VOLTAGE A In asynchronized pressure ink jet system of the type described herein,the-drop breakoff and charge voltage timing must by synchronized. Thisrequires thatthe charge voltage applied by electrode 18 shall have mentof ,drop formation time with respect to drop charging through anelectronic feedback loop.

*At anytime when drops 2 are not being used to print characters on paper1, such as atthe home position, between characters, or during deflectionof drops on a column by column basis, a sampling function'is performed.Thus, synchronization can take place any time that printing is nottaking place.

Reference is made to FIG. 3 that shows typical time intervals and pulsewave forms during a single drop time, assumed to be 10 microseconds induration. The first portion of the total of i microseconds, that is, 2.5microseconds, is set aside to allow the driver time to reach the propercharging voltage. The remainder of he. rqp ti srthat sr .7.- Iin !999nd.m-gr ized as the safe area of drop formation. Accordingly, a drop formedany time during this interval should be properly charged and deflectedas it is propelled toward paper 1.

SYNCHRONIZATION WITH NARROW PULSES GUTTER NO. 1

Several wave forms are of interest in FIG. 3. As one example, a seriesof narrow pulses on line 41 may be directed to driver 21 undercontrolled conditions. Sync control 40 adjusts driver 15 voltage througha range of values until the narrow pulseseffect charging of drops 2'.During synchronization using gutter No. 1, pulse signals are appliedfrom Sync Control block 40 by line 41 to charge electrode driver 21 andultimately to charge electrode 18. As shown in FIG. 3, a typical pulsevoltage range is from 0 volts to volts. Machine logic 13 by line 44switches analog analog switch 46 to receive an input only from gutter 35on line 47 designated A." Ordinarily, a sequence of drops moves intogutter No. 1 designated 35 developing a currentzof some level directlyproportional to the amount of charge on a series of'individual drops.Such a series may comprise a portion of a column of unused drops orseveral such column s, depending on system sensitivity. Means, known inthe art, are mounted in association with gutter N o. l to provide acurrent level on line 47 corresponding to and directly related to thecharge levels on drops 2 going into gutter No. 1. As an example of onepossible configuration, for every two volt difference potential appliedonto a series of drops 2 by charge electrode 18, approximately onenanoampere of current is derived on line 47. If for example", drops in aseries were charged by 10 volts on the charge electrode, on average,then 5 nanoamperes is available on line 47. The output of switch 46 isdirected to a drop charge amplifier 50 that converts the current levelsto a usable voltage level. The output of amplifier 50' is directed byline 30 52 along with reference 'voltage from source; 54 on line 55 forcomparison in a comparator circuit 57. The output of comparator 57on-line 60 isdirected to Sync Control 40 which concurrently is supplyinga series of stepped driving voltage levels by line 62 to crystal 3driver 15, thereby adjusting the timing of drop separationandmaintainingthe time of drop separation in a desired relationship withrespect to the potential applied by driver2l. Synchronization isattained when charged drops are first detected. It is also possible tosynchronize with narrow pulses using gutter No. Zdesignated 36. Y l

* SYNCI-IRQNIZATION WlTI-IRAMP SIGNAL Y i ,HGUYTTER Nor 1 As a preferredalternative, a ramp signal ranging from 0 to l0volts is provided .byline.42 to driver 21.

Synchronization is attained using gutter No. '1 desig electrode 18.Machine logic 13 by line 44 switches analog switch 46 to receive aninput only from gutter 35 on line 47 designated A. As before, a sequenceof drops moves into gutter No. 1 designated 35 developing a current ofsome level directlyproportionalto the amount of charge on a series ofindividual drops. Such a series may also comprise a portion of acolumn'of unused drops or several such columns, depending on systemsensitivity. A current level is developed on line 47 corresponding toand directly related to the charge levels on drops 2 going into gutterNo. 1. The current level will depend upon the time of drop breakoffduring the ramp signal interval. In this case, as before, the output ofswitch 46 is directed to a drop charge amplifier 50 Duringsynchronization using gutter No. 1, the signal is applied from'SyncControl block 40 by line 42 to charge electrode driver 21 and ultimatelyto charge SYNCHRONIZATION WITH RAMP SIGNAL GUTTER NO. 2

Gutter No. 2 designated 36 in FIG. 1 may also be used'for' normal dropsynchronization of phasing, if desired. A synchronization pulse usinggutter No. 2 is illustrated as a ramp wave form in FIG. 3, and issupplied on line 81 from Sync Control 40 to driver 21. Such checkingwith gutter No. 2 is evidence that proper character height is beingmaintained. This may also be pulse may, for example, be in the range of200 to 250 7 volts. It is noted that a greater change that is, 50 volts,occurs in this case than in the case 'of synchronization using gutterNo. l, where only a 10 volt change occurs. The greater rate of changewhensynchronization is effected with gutter No. 2 provides a much higherlevel of current that is easier to detect in the circuits.

In any of the foregoing instances, if a sufficient series of drops 2passes through the selected gutter, a current in nanoamperes isdeveloped that is detectable. Amplifier 50 convertsits input currentlevel to low impedance voltage level. If a ramp is used, and, as anexample, reference voltage supply 54 indicates 5.0 volts as a desiredvoltage level, and amplifier 50 provides a 7.5 volt level toco.mparator.57, a corrective signal is developed on line 60 to changethedrive voltage applied by driver under control of Sync Control 40 inorder to more closely achieve the 5.0 volt level. It is somewhatimpracticalto attempt to sense the current levels developed by a singleink drop aswould be done in a 100 percent servo system. Thus, a seriesof drops is integrated to develop the'necessary current levels. Such aseries of drops would be available, as an example, when an entire columnof droplets or most of the droplets in a column are available duringprinting of characters of symbols.

The foregoing synchronization procedures and feedback circuit's aregated on by machine logic 13 during any non-printing time, as desired,and may occur when the machine is first turned on, when the firstcharacter is printed, in between characters, in between lines, etc.

USE OF CHECKING PULSE GUTTER NO. 1 A checking pulse may be applied online 43 ha low voltage range such as 0l0 volts which when sensed bycurrent levels on line 47 can determine various system failure, such asdrop charging failure, loss of synchronization, etc. However, for a'mostcomplete check, gutter No. 2 is preferably used.

USE OF CHECKING PULSE AND SYSTEM FAILURE DETECTION USING GUTTER NO. 2

done while printing is taking place whenever unused drops occur. When inchecking mode B," machine logic l3 forces switch 46 to pass current onlyfrom input b connected to gutter N0. 2, 36, by ling 74. As an example, asequence of drops in a given series of drop intervals could besufficient to develop adequate current on line 74.

. The 10 volt level used for synchronization into gutter No. l developsa gutter current on line 47 in the range of 4 to 6 nanoamperes. Agreater charge level required to deflect drops 2 up to gutter 36achieves a higher current level that is easier to sense by circuits 46and 50.

If, for example, volts deflects drops 2 to the upper portion of paper 1during printing of chaacters, to 200 volts applied to unused drops candeflect those drops to gutter 36. Such voltage levels develop currentlevels in gutter 36 and on line 74 in the range of 90 to 100 nanoampereswhich is about 20 times the current levels developed on line 47. It isapparent that the higher 'current levels are more readily sensed andworked with in the circuits as shown.

The advantage of the checking pulse applied over a number of dropintervals is that the system can determine whether propersynchronization is being maintained, whether the charge electrode anddriver are op erating properly, whether the high voltage fromterminal'2'5 is present and of course, whether other components in thesystem are working properly. The checking pulses are applied by line 65to driver 21 in order to produce the required currents in gutter 36.These may be compared by comparator 57- against reference voltage fromblock 54 with suitable changing of reference level on line 77 from SyncControl 40.

A possible approach using this technique is for machine logic 13 todetermine the maxium number of drops used during printing of charactersand thus also determine the minimum'number of drops that should beavailable for gutter No. 2 during normal printing. Such minimum numberof drops should result in a particular current level on line 74 which ismeasured by circuits 46 and 50 and compared at comparator 57 with athreshold level from block 54. If such minimum potential is notavailable, a failure is indicated to machine logic 13 on line 80. Themachine logic can then take steps to turn off the crystal drive fromdriver 15-, to turn off driver 21, deactivate pumps 30 and 6 and takeany othersteps necessary to minimize the amount of ink that is sprayedon paper 1 or other parts of the mechanism.

The foregoing techniques result in less costly measuring circuitryand-less-noise sensitive signals. An important advantage of the checkingprocedure is that the logic 13 is able to continually monitor theperformance of the ink jet mechanism, if desired.

ALTERNATIVE CHECKING CIRCUIT FIG. 2 illustrates an alternative checkingcircuit having various members 35, 36, 46, 47, 50 and 74 correspondingto those similarly numbered members in FIG. 1, but having a comparator57a and an independent second reference source 54a that is basedstrictly on a go-no-go basis. The analog switch 46 monitors dropsZimpinging on gutter No. 2 (36) and essentially determines if the dropsare present or absent. Reference voltage 2 from source 54a establishes athreshold level for comparator 57a and the output from amplifier 50should always exceed such level. If it does not then a signal isprovided by line 80a to indicate a machine failure.

to decode selector 84, on line 85 to printing logic 86 and on line 87 tocircuit 88. Sync time is indicated by printing logic 86 on lines 90 and91 that are directed to Sync Control 40a and And circuit 88,respectively. The circuit includes a crystal 95 receiving ink on line 96from an ink supply, not shown, a nozzle 98 and a crystal driveramplifier 99. Amplifier 99 drives crystal 95 to form drops 100 that passthrough charging electrode 102 driven by charge electrode driver 104.Driver 104 in turn is driven from Or circuit 88. Individual drops aredeflected to paper 1, for printing or to gutter 106 when not used. Thecircuit includes a counter register 110 that serves during normalprinting times to define the crystal driver signal.

During synchronization time, Sync Control 40a advances counter'register110 until a charge current above a predefined level is found. This isdone by detection of current levels in gutter 106, supply of such levelson line 112 to amplifier (AMP) 50, conversion of such levels to voltagelevels amplifier 50 and comparison of such levels with reference voltagefrom terminal 113 by comparator (COMP) 114. The output of comparator 114is directed to Sync Control 40a. When a charge current above apredefined level is recognized, the value in counter 110, is held. Thismaintains the drive level from amplifier 99 until the nextsynchronization time. Counter register 110 outputs are decoded by block84 which in turn is driven from master clock 82. The individual clocksignals each have a different phase relationship with respect to thesynchronization pulse on line 87. If Sync Control block 40a is unable tofind a signal which provides synchronization it will so indicate by line115 to printing logic 86 thereby evidencing a synchronization failure.The Sync-Control logic contains a delay-to. compensate for the transittime ofthe drops.

The sync time signal produced by the printing logic 86 controls gate 88so as to allow only printing signals to go to the charge electrodedriver dring printing and allow only sync pulses to reach the chargeelectrode driver 104 during synchronization.

While the invention has been particularly shown and described withreference with several embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madewithout departing fr m he spiri a d.299P 9f,th.. n.vsmi9n- 'What isclaimed is:

1. An ink jet printing system for printing on a record mediumby ink dropdeposition, comprising: means for supporting said record medium .at aprinting station; means for forming and propelling ink drops toward saidrecord medium; i cyclically operable means operable in timed relationwith said forming and propelling means for charging and deflecting aplurality of said drops on a selective basis to form visible indiciaupon said rec qm s har n .s qifl sflife,

being operable to charge and deflect said drops in a range from arelatively low to high potential with the extremes of said rangerepresenting drops not required for printing; first gutter meanspositioned in the path of travel of a first type of unused drops chargedat said relatively low potential to collect said first type of dropssaid first gutter means providing a relatively low synchronizationcurrent from charged drops travelling in said first gutter means secondgutter means positioned in the path of travel of a second type of unuseddrops charged at said relatively high potential to collect said secondtype of drops said second gutter means providing a relatively highchecking current level; from charged drops travelling in said secondgutter means; amplifier means interconnected with said first guttermeans for providing a low synchronization potential responsive to therelatively low synchronization current provided by said first guttermeans, and further interconnected with said second gutter means forproviding a high checking potential responsive to the high checkingcurrent level from said second gutter means; character generating meansfor activating said charging and deflecting means in a printing rangewithin but not including said relatively low and relatively highpotential in order to deflect selected ones of said drops within aprintin range on said record medium; synchronization means operable toactivate said charging and deflecting means at said relatively lowpotential in order that low synchronization potential is developed bysaid amplifier means; checking means operable to activate said chargingand deflecting means at said relatively high potential in order that thehigh checking potential. is developed by said amplifier means; comparingmeans for comparing said synchronization potential withpredetermined'reference levels representative of correct synchronizationin said systemyw means controlled by said comparing means for main- Ataining correct phase between drop formation and charging; saidcomparing means comparing said checking potential with predeterminedreference levels representative of correct operation of said system; andoutput means from said comparing means for indicating a failure of saidsystem. 2. The system of claim 1 further comprising: means mounting saidforming and propelling means, said charging and deflecting means andsaid first and second gutter means in a unitary structure adjac ent saidrecord medium; means operable in timed relation to drop propulsiontoward said record medium for relatively moving said mounting means andsaid record medium from a home position in order to form matricesarranged in lines printed intelligence on said record medium each ofsaid matrices comprising a set of vertical columns and horizontal rowsof drop locations and each of said matrices being separated from apreceding or succeedingmatrix by an inter-matrix gap. 3. The system ofclaim 2 wherein:

said synchronization control means provides synchronization pulses whensaid mounting means and said record medium are relatively positioned athome position; and

checking pulses any time during relative movement when drops are notrequired for printing.

4. The system of claim 1 further comprising:

an analog switch responsive .to current levels from said first sensingmeans when activated in a first mode and to current levels from saidsecond sensing means when activated in a second mode;

selection means for selecting said first and second modes of said switchon a selective basis;

amplifier means responsive to current levels from said analog switch todeveloppotentials therefrom, and

means connecting potentials from said amplifier means to said comparingmeans.

5. The system of claim 1 further comprising:

a charge electrode and charge electrode driver asso- 10 ciated with saiddrop charging and deflecting means;

a crystal and crystal driver associated with said drop forming andpropelling means; and

means applying synchronization and checking signals from saidsynchronization control means to said charge electrode through saidcharge electrode driver;

means'for applying output signals from said first comparing means tosaid synchronization control means;

and means in said synchronization control means for developing andapplying correction signals to said crystal through said crystal driverin order to maintain in proper phase.

6. The system of claim 1 further comprising:

means responsive to a failure detection signal from said comparing meansto de-activate said system.

1. An ink jet printing system for printing on a record medium by inkdrop deposition, comprising: means for supporting said record medium ata printing station; means for forming and propelling ink drops towardsaid record medium; cyclically operable means operable in timed relationwith said forming and propelling means for charging and deflecting aplurality of said drops on a selective basis to form visible indiciaupon said record medium, said charging and deflecting means beingoperable to charge and deflect said drops in a range from a relativelylow to high potential with the extremes of said range representing dropsnot required for printing; first gutter means positioned in the path oftravel of a first type of unused drops charged at said relatively lowpotential to collect said first type of drops said first gutter meansproviding a relatively low synchronization current from charged dropstravelling in said first gutter means; second gutter means positioned inthe path of travel of a second type of unused drops charged at saidrelatively high potential to collect said second type of drops saidsecond gutter means providing a relatively high checking current level;from charged drops travelling in said second gutter means; amplifiermeans interconnected with said first gutter means for providing a lowsynchronization potential responsive to the relatively lowsynchronization current provided by said first gutter means, and furtherinterconnected with said second gutter means for providing a highchecking potential responsive to the high checking current level fromsaid second gutter means; character generating means for activating saidcharging and deflecting means in a printing range within but notincluding said relatively low and relatively high potentials in order todeflect selected ones of said drops within a printing range on saidrecord medium; synchronization means operable to activate said chargingand deflecting means at said relatively low potential in order that lowsynchronization potential is developed by said amplifier means; checkingmeans operable to activate said charging and deflecting means at saidrelatively high potential in order that the high checking potential isdeveloped by said amplifier means; comparing means for comparing saidsynchronization potential with predetermined reference levelsrepresentative of correct synchronization in said system; meanscontrolled by said comparing means for maintaining correct phase betweendrop formation and charging; said comparing means comparing saidchecking potential with predetermined reference levels representative ofcorrect operation of said system; and output means from said comparingmeans for indicating a failure of said system.
 2. The system of claim 1further comprising: means mounting said forming and propelling means,said charging and deflecting means and said first and second guttermeans in a unitary structure adjacent said record medium; means operablein timed relation to drop propulsion toward said record medium forrelatively moving said mounting means and said record medium from a homeposition in order to form matrices arranged in lines printedintelligence on said record medium each of said matrices comprising aset of vertical columns and horizontal rows of drop locations and eachof said matrices being separated from a precedIng or succeeding matrixby an inter-matrix gap.
 3. The system of claim 2 wherein: saidsynchronization control means provides synchronization pulses when saidmounting means and said record medium are relatively positioned at homeposition; and checking pulses any time during relative movement whendrops are not required for printing.
 4. The system of claim 1 furthercomprising: an analog switch responsive to current levels from saidfirst sensing means when activated in a first mode and to current levelsfrom said second sensing means when activated in a second mode;selection means for selecting said first and second modes of said switchon a selective basis; amplifier means responsive to current levels fromsaid analog switch to develop potentials therefrom, and means connectingpotentials from said amplifier means to said comparing means.
 5. Thesystem of claim 1 further comprising: a charge electrode and chargeelectrode driver associated with said drop charging and deflectingmeans; a crystal and crystal driver associated with said drop formingand propelling means; and means applying synchronization and checkingsignals from said synchronization control means to said charge electrodethrough said charge electrode driver; means for applying output signalsfrom said first comparing means to said synchronization control means;and means in said synchronization control means for developing andapplying correction signals to said crystal through said crystal driverin order to maintain in proper phase.
 6. The system of claim 1 furthercomprising: means responsive to a failure detection signal from saidcomparing means to de-activate said system.